Various processes are employed in the pulp and paper industry to bleach wood pulp. Among these processes are peroxide bleaching processes and thiourea dioxide bleaching processes.
Peroxide is an effective lignin-preserving bleaching agent. It improves the brightness of groundwood, i.e. mechanical, chemimechanical or other highly lignified pulps, without significant shrinkage in yield. Peroxide may be provided by conventional inorganic peroxides such as hydrogen peroxide or sodium peroxide, or by organic peroxides such as benzoyl peroxide, di(tertiary-butyl) peroxide or peracetic acid. Peroxide bleaching solutions, usually with hydrogen peroxide, are generally adjusted to a pH of about 9 to 11 by addition of sodium silicate, and optionally, sodium hydroxide, before addition to a pulp slurry. Such solutions may also contain a small amount of a soluble magnesium salt, e.g., magnesium sulfate, as a stabilizing agent and an organic chelating agent such as diethylenetriamine pentaacetic acid or alkali metal salts thereof. In peroxide bleaching, pulp consistency in the aqueous slurry is generally between about 1 and about 35 percent by weight. The process is usually conducted at temperatures between about 30.degree. C. and 90.degree. C., and retention times are generally from about 1 to 5 hours. The strength of the peroxide bleaching solution expressed in terms of its peroxide content is usually in the range of about one to three percent peroxide by weight. Higher strengths may be used but are economically unattractive. Following peroxide bleaching, residual peroxide generally remains in the pulp slurry. Standard practice in the bleaching industry has been to neutralize residual peroxide with either sulfur dioxide or sodium bisulfite.
The aforesaid neutralization process requires use of a reagent such as sulfur dioxide to neutralize residual peroxide in the pulp slurry. Such a process is wasteful of both the excess peroxide and the neutralizing agent. It has now been found that if thiourea is added to a wood pulp slurry containing residual peroxide following a peroxide bleaching step, additional bleaching of the wood pulp can be achieved beyond that provided by peroxide bleaching.